Deus Ex is just $5. Graphically the game has not aged at all, but gameplay it's still pretty solid. Probably one of the best FPS out there, and an amazing soundtrack too.

Hotline Miami for $5. I like to call it an action puzzle game. Style and gore out the wazoo. Set in the gritty 80s. Very fun and rewarding game once you figure out the solution. Your final run is usually a swift perfect execution which feels really satisfying.